From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.9 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 40766C2D0A3 for ; Mon, 9 Nov 2020 08:53:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DCD22206ED for ; Mon, 9 Nov 2020 08:53:21 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="ez1kxHo5" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726127AbgKIIxV (ORCPT ); Mon, 9 Nov 2020 03:53:21 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:24571 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728335AbgKIIxV (ORCPT ); Mon, 9 Nov 2020 03:53:21 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1604911999; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=HNQPsjTQl5LJpWlmaOSDh69FhP0/dMoZ5uSD4dWGZEs=; b=ez1kxHo5mFW+6qJ2nrmFW+zolr3+u/fjc6RpWFOYUHtrm1YOosDAbGkpbHGZ78JJPT5no2 5LA5ZJHDp3D2/rKdMPt0z62yVYy78g3muWqzxgGA9JGoE3IiFSI8ZE/i8givrkVL2tQGL3 TaDNfdtyuVlNypU8emJpnmh/Rz9EVWk= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-249-w_uVmLk2NUOGd9-mA0g_Ww-1; Mon, 09 Nov 2020 03:53:15 -0500 X-MC-Unique: w_uVmLk2NUOGd9-mA0g_Ww-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id DD64D1009E2D; Mon, 9 Nov 2020 08:53:13 +0000 (UTC) Received: from localhost (ovpn-113-223.ams2.redhat.com [10.36.113.223]) by smtp.corp.redhat.com (Postfix) with ESMTPS id CFEE05C1D7; Mon, 9 Nov 2020 08:53:08 +0000 (UTC) From: Giuseppe Scrivano To: Amir Goldstein Cc: Vivek Goyal , Sargun Dhillon , overlayfs , Miklos Szeredi , Daniel J Walsh Subject: Re: [PATCH v7] overlayfs: Provide a mount option "volatile" to skip sync References: <20200831181529.GA1193654@redhat.com> <20201106190325.GB1445528@redhat.com> <87o8kamfuo.fsf@redhat.com> Date: Mon, 09 Nov 2020 09:53:01 +0100 In-Reply-To: (Amir Goldstein's message of "Sat, 7 Nov 2020 11:35:04 +0200") Message-ID: <87k0uulxn6.fsf@redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Precedence: bulk List-ID: X-Mailing-List: linux-unionfs@vger.kernel.org > On Fri, Nov 6, 2020 at 9:43 PM Giuseppe Scrivano wrote: >> >> Vivek Goyal writes: >> >> > On Fri, Nov 06, 2020 at 09:58:39AM -0800, Sargun Dhillon wrote: >> > >> > [..] >> >> There is some slightly confusing behaviour here [I realize this >> >> behaviour is as intended]: >> >> >> >> (root) ~ # mount -t overlay -o >> >> volatile,index=off,lowerdir=/root/lowerdir,upperdir=/root/upperdir,workdir=/root/workdir >> >> none /mnt/foo >> >> (root) ~ # umount /mnt/foo >> >> (root) ~ # mount -t overlay -o >> >> volatile,index=off,lowerdir=/root/lowerdir,upperdir=/root/upperdir,workdir=/root/workdir >> >> none /mnt/foo >> >> mount: /mnt/foo: wrong fs type, bad option, bad superblock on none, >> >> missing codepage or helper program, or other error. >> >> >> >> From my understanding, the dirty flag should only be a problem if the >> >> existing overlayfs is unmounted uncleanly. Docker does >> >> this (mount, and re-mounts) during startup time because it writes some >> >> files to the overlayfs. I think that we should harden >> >> the volatile check slightly, and make it so that within the same boot, >> >> it's not a problem, and having to have the user clear >> >> the workdir every time is a pain. In addition, the semantics of the >> >> volatile patch itself do not appear to be such that they >> >> would break mounts during the same boot / mount of upperdir -- as >> >> overlayfs does not defer any writes in itself, and it's >> >> only that it's short-circuiting writes to the upperdir. >> > >> > umount does a sync normally and with "volatile" overlayfs skips that >> > sync. So a successful unmount does not mean that file got synced >> > to backing store. It is possible, after umount, system crashed >> > and after reboot, user tried to mount upper which is corrupted >> > now and overlay will not detect it. >> > >> > You seem to be asking for an alternate option where we disable >> > fsync() but not syncfs. In that case sync on umount will still >> > be done. And that means a successful umount should mean upper >> > is fine and it could automatically remove incomapt dir upon >> > umount. >> >> could this be handled in user space? It should still be possible to do >> the equivalent of: >> >> # sync -f /root/upperdir >> # rm -rf /root/workdir/incompat/volatile >> > > FWIW, the sync -f command above is > 1. Not needed when re-mounting overlayfs as volatile > 2. Not enough when re-mounting overlayfs as non-volatile > > In the latter case, a full sync (no -f) is required. Thanks for the clarification. Why wouldn't a syncfs on the upper directory be enough to ensure files are persisted and safe to reuse after a crash? Regards, Giuseppe